This application is based upon Japanese Patent Application No. Hei 8-212325 (JP-A-10-56131), filed Aug. 12, 1996, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
This invention relates to a semiconductor device comprising one or more semiconductor chips such as for example power MOSFETs and IGBTs built into a package.
2. Description of Related Art
Because semiconductor chips such as power MOSFETs and IGBTs are devices for controlling large currents, they produce large amounts of heat. Consequently, when these semiconductor chips are built into packages, it is arranged to achieve a sufficient cooling (heat radiation). For example, in the case of an IGBT module consisting of a plurality of IGBT chips built into a package, an insulating substrate made of a high thermal conductivity ceramic is used, and the plurality of IGBT chips are mounted on this insulating substrate, and main electrodes provided on the lower surfaces (lower principal surfaces) of the IGBT chips are connected by soft soldering to a copper thick film provided on the insulating substrate.
Main electrodes and control electrodes provided on the upper surfaces (upper principal surfaces) of the IGBT chips are connected to a copper thick film provided on the insulating substrate by wire bonding. The insulating substrate is soldered to a heat radiation plate made of copper. By. this means, heat produced by the IGBT chips is transmitted through the insulating substrate to the heat radiation plate and radiated away. This kind of IGBT module is used in invertor main circuits of invertors in a class of several tens to several hundreds of Amperes.
In the case of an IGBT module of the related art construction described above, heat produced by the IGBT chips is radiated through the insulating substrate disposed on the lower surface side of the IGBT chips; that is, heat is radiated mainly from the lower surface of the IGBT chip. With this construction, because heat is only radiated from one surface of each of the IGBT chips, there is a limit to how much the heat-radiation performance can be raised, and reducing the size of the construction of the IGBT module as a whole has also been difficult.
When on the other hand the IGBT module is constructed so that heat is radiated from both the upper surface and the lower surface (the two principal surfaces) of the semiconductor chips, the heat-radiation performance can be increased greatly. One example of this construction is a thyristor package. This package has a structure wherein a thyristor chip is sandwiched by two electrode blocks serving both as electrodes and as heat sinks (radiator). With this construction, heat produced by the thyristor chip is transmitted from both the upper surface and the lower surface of the chip to the electrode blocks. In the case of a thyristor, to obtain electrical connection between the electrodes of the thyristor chip and the electrode blocks, the thyristor chip sandwiched by the electrode blocks is pressed with a considerably large force.
However, semiconductor chips like IGBT chips having MOS gate structures have the characteristic that they are vulnerable to stresses. Consequently, it is not possible to employ the method of pressing the semiconductor chips with electrode blocks To overcome this, as a construction wherein semiconductor chips are sandwiched between two high thermal conductivity insulating substrates without being pressed, there is the construction disclosed in Japanese Patent Application Laid-Open No. S.59-31042. In this case of this Japanese Patent Application Laid-Open No. S.59-31042, because the lower side of the semiconductor chip is fixed to an electrode provided on an insulating Substrate, heat produced by the semiconductor chip is radiated from this lower side of the chip smoothly.
However, on the upper side of the semiconductor chip, because the electrodes on this upper side and electrodes provided on the upper insulating substrate are connected by bonding pads and metal bumps, the area of the connection is small. Consequently, there has been the problem that the electrical resistance is large, which is disadvantageous to obtaining large currents, and that heat produced by the semiconductor chip is not readily transmitted to the insulating substrate, and thus the heat-radiation performance is poor.
The present invention is made in light of the foregoing problems, and it is an object of the present invention to provide a semiconductor device which improves heat radiation performance; which can be reduced in size; and with which heat can be radiated swiftly from two principal surfaces of a semiconductor chip even if the semiconductor chip has a construction such that it is vulnerable to stresses.
According to a semiconductor device of the present invention, a semiconductor chip is sandwiched between two high thermal conductivity insulating substrates, and the electrodes of the semiconductor chip and electrode patterns on the high thermal conductivity insulating substrates are bonded by brazing. Consequently, heat produced by the semiconductor chip is smoothly transmitted from the two principal surfaces of the semiconductor chip to the two high thermal conductivity insulating substrates, and is thereby radiated quickly. As a result, it is possible to reduce the size of the semiconductor device. Also, because the electrodes of the semiconductor chip and the electrode patterns on the high thermal conductivity insulating substrates are bonded by brazing, the semiconductor chip is not required to be pressed, and furthermore, the bonding area (connection area) becomes larger to decrease its electrical resistance and heat resistance.
According to another aspect of the present invention, a plurality of semiconductor chips having two principal surfaces front-rear reversed with respect to each other are sandwiched between the two high thermal conductivity insulating substrates.
Therefore, the shape of electrode patterns on the high thermal conductivity insulating substrates can be simplified.
Furthermore, the high thermal conductivity insulating substrates may be made of aluminum nitride. In this case, because the coefficient of thermal expansion of aluminum nitride is close to that of the silicon constituting the semiconductor chip, it is possible to reduce thermal stresses acting between the semiconductor chip and the electrode patterns.
Furthermore, the height of bonding parts of the electrode patterns of the high thermal conductivity insulating substrates, that is, parts to be bonded to the electrodes of the semiconductor chip, may be made higher than that of non-bonding parts, and the sizes of these bonding parts may be made the same as or smaller than the sizes of the respective electrodes of the semiconductor chip. Accordingly, it is possible to prevent runaround of solder and to avoid a guard ring of the semiconductor chip in the bonding.